Various types of bow guides for a variety of bow played string instruments are known in the prior art. Some are quite complex. Some must be fastened to the string instrument, a trait that is undesirable for several reasons. As example, fastening a bow guide to the string instrument may physically damage the string instrument. Fastening the guide to the string instrument also typically significantly alters tone values. And, fastening the guide to an string instrument is time consuming in both fastening and unfastening as needed. What is needed is a string instrument bow guide that selectively fits a string instrument quickly, without being fastened to the string instrument. What is further needed is a bow guide that does not harm the string instrument in any way or significantly alters the tone of the string instrument. And, a bow guide is needed that provides a somewhat universal fit to a class of string instruments. Further, a bow guide is needed that is selectively wholly or partly disassembled with ease. Also, a bow guide is needed that is selectively fitted within a string instrument case and therein be carried along with the string instrument rather than separately. The present string instrument bow guide provides all of the advantages.